The present invention relates to a communication system for communicating a speech signal confidentially and particularly to an improvement in reducing the selective fading of the confidential communication system which transmits speech feature parameters instead of speech signal itself.
The confidential communication system transmits spectrum envelope information representing macroscopic spectrum distribution and exciting source information representing microscopic spectrum distribution as speech feature parameters, and is widely used in various fields because of its high capability of secrecy.
Such speech feature parameters are transmitted through a plurality of frequency sub-bands (multiple paths) of radio frequency band by utilizing low-bit rate CODEC such as a vocoder, thereby reducing the problem in the transmission channels.
In the above-mentioned conventional confidential communication system, however, the multiple paths are formed through the reflection from the ionosphere. Therefore, in the communication channel there may be caused a selective fading in a frequency band sufficiently narrow compared with the speech band, thereby resulting in multiple null receptions by mutual interference of multipath-transmitted signals. Moreover, in the mobile communication system, the null frequency moves randomly at high speeds depending on the path difference and the carrier frequency, making it impossible to use such a system except under only limited operating conditions.